Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the pyrolysis furnace for further downstream processing.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules that tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contains a large fraction of light volatile hydrocarbons, such as ethane, and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, gas oil, crude oil and atmospheric residue. Gas oil, crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of about 590° C. (1100° F.) otherwise known as resids.
Cracking heavier feeds, such as residues, kerosenes and gas oils, produces large amounts of tar, which typically contains high-boiling and/or non-volatile components including paraffin-insoluble compounds, such as pentane-insoluble (PI) compounds or heptane-insoluble (HI) compounds, which are molecules of high molecular weight with multi-ring structures, e.g., asphaltenes. These materials reduce the economic value of tar by rendering it highly viscous and less compatible for mixing with highly paraffinic streams, inducing precipitation of the paraffin-insoluble components from the resulting mixture.
Various methods are known in the art to treat tars produced from steam cracking.
U.S. Pat. No. 3,691,058, incorporated herein by reference in its entirety, discloses an integrated visbreaking-hydrocracking process to break down steam cracked tars into single-ring aromatics.
U.S. Pat. No. 3,707,459, incorporated herein by reference in its entirety, discloses visbreaking residua, e.g., thermal tar from steam cracking, in the presence of free radical acceptors, e.g., CaO, isooctane, and n-heptane.
U.S. Pat. No. 4,575,413, incorporated herein by reference in its entirety, discloses adding aluminum salts to reduce fouling in steam cracked tar streams.
DE 4308507 discloses reducing viscosity of heavy oil residues by treatment at high temperature (427° C.) with a hydrogen donor solvent comprising a fuel oil from steam cracking, which contains hydroaromatic compounds.
U.S. Pat. No. 5,215,649, incorporated herein by reference in its entirety, discloses producing gaseous olefins by cracking a hydrocarbon feedstock stream wherein the cracked product stream is quenched to stop cracking, followed by injecting hydrogen donor diluent, e.g., dihydronaphthalenes, which suppress molecular weight growth reactions forming undesirable high molecular weight materials such as asphaltenes. The '649 patent (paragraph bridging columns 5 and 6) teaches steam cracked liquid product contains “free radical molecules, vinyl-aromatic molecules, and other reactive species, and is highly reactive at moderately high temperatures commonly found in the downstream processing of steam cracked liquid product.” Steam cracked product is cooled in a heat soaking vessel to a temperature of 149° to 401° C. (300° to 755° F.) and hydrogen donor diluent is introduced to the heat soaking vessel at a temperature within 260° to 482° C. (500° to 900° F.) (column 6, lines 34 to 66). Quench oil can be added with the hydrogen donor diluent to form a quenched mixture temperature within the range of 260° to 343° C. (500° to 650° F.) (column 6, lines 59 to column 7, line 10).
It would be desirable to provide an apparatus and process to convert steam cracker tar to more valuable, lower boiling materials. Moreover, it would be particularly desirable to provide such an apparatus and process which are self-contained, treating steam cracker tars without adding additional materials such as hydrogen, hydrogen donors, or aluminum compounds.